Abstract: Diurnal patterns of non-structural carbohydrates and relative enzymes in mature leaves and phloem sap in cucumber (Cucumis sativus L. ) were studied. The results showed that glucose and fructose are primary soluble sugars from night to 10: 00 a.m. in mature leaves, but during 13: 00 - 16: 00 p.m. , stachyose becomes the most abundant oligosaccharide, this diurnal change is mostly due to the tempestuous fluctuation of stachyose concentration in mature leaves. During photoassimilate transportation, the p rimary soluble sugar in phloem sap is stachyose. When transportation is finished, sucrose becomes the main sugar in phloem sap. Cucumber leavesmay store photoassimilate in the forms of starch, glucose, fructose, stachyose and sucrose at night. The concentrations of glucose, fructose and stachyose in phloem sap are much lower than in leaves at that time. There may be some unique mechanisms modulating source-sink translocation by which soluble sugars in leaveswere prevented from loading into the phloem further during night.

Key words: Cucumber, Carbohydrate, Diurnal change